Oncolytic virus (OV) therapies for cancer employ a dual mechanism of action and have two major advantages. First, OV are attenuated and designed to efficiently replicate in and kill only cancer cells, creating a large safety profile. Second, as observed in animal models and human trials, OV replication and spread through tumor tissue programs the immune system to target distant metastases for elimination. This dual mechanism of action combines elements of the most effective traditional and emerging therapeutic modalities: selective cancer cell cytotoxicity and tumor vaccination. In this application, we propose to evaluate BV-2711, a novel Herpes Simplex Virus Type 1 (HSV1) OV. BV- 2711 utilizes the same oncolytic mechanism of action as OncoVEXGMCSF, but also encodes an endogenous HSV1 immune evasion function. In the Phase 2 trial of OncoVEXGMCSF in advanced metastatic melanoma, 26% of patients responded to therapy. Importantly, in all responders, uninfected metastases were reduced or disappeared entirely due to a virus-induced tumor vaccination effect. 52% of all responders were still alive when the Phase 2 results were published, more than two years after treatment initiation. The oncolytic mechanism used in OncoVEXGMCSF results in the deletion of the HSV1 Us12 gene, which blocks viral antigen display on the cell surface by MHC Class I. Us12-deficient HSV1 are known to be prematurely cleared by CD8+ T-cells. This likely resulted in premature clearance of OncoVEXGMCSF and limited its efficacy in the Phase 2 trial. Evasion of CD8+ T-cells could improve HSV1 OV efficacy, given the high seroprevalence of HSV1 in the population. It is our hypothesis that restoration of the endogenous Us12 gene, as in BV-2711, will lead to increased tumor reduction and tumor vaccination efficacy. Preliminary data in an immunocompromised mouse model of human bladder cancer demonstrated that the oncolytic properties of BV-2711 are equivalent to an OncoVEXGMCSF surrogate. However, because these mice lack T-cells, the relevance of CD8+ T-cell evasion to OV efficacy remains undetermined. In this application, we propose to test our hypothesis in immune competent mouse models of bladder cancer. Aim 1. Construct and evaluate a panel of BV-2711-based OV optimized for immune-competent mouse models. Aim 2. Evaluation of BV-2711 variants in syngeneic mouse model of bladder cancer. PUBLIC HEALTH RELEVANCE: Many forms of bladder cancer have limited options for therapy. In this proposal, we will develop and evaluate a virus for bladder cancer therapy that specifically targets cancer cells and uses the immune system to help in their clearance. We believe this could be safer and more effective that existing treatments.